I don't think a zoomed UTH will melt a plastic tub. It might warm it or disfigure it in some way, but 120* will not melt it.

Either way, needs to be used with a thermostat.

I have been using a reptitemp 500r with one piece of flexwatt (single setup) going on 2 years now. I see about a 1-2* temperature fluctuation.